This invention concerns a multilayer anti-corrosive coating for the entire surface of a ferrous metal element, such as a water system pipe.
Petroleum bitumen-based coatings are known and used as internal and external coatings for water system elements. These coatings are low-cost but require the use of solvents, which increases the risk of fire.
In other known coatings, all the layers are obtained from epoxy powder-based compositions, which offer good corrosion resistance. But the thickness of such coatings varies, especially at the joints where the elements are dimensioned to fit together for assembly (seal grooves, centering diameters), thereby making it necessary either to increase the number of foundry models with different dimensions for the different coatings, or to machine down the metal elements prior to coating.
These coatings consist of inert material forming a barrier against external elements. This barrier, however thick it may be, is still subject to impact damage that will bare the substrate, at which point protection no longer exists, without involving any chemical reactions between the coating and its substrate. This is true of both petroleum bitumen and epoxy powder. In order for this protection to have maximum effectiveness, it is important that the bitumen or epoxy be free of damage down to the metal.
If these conditions are not met, corrosion sets up electrical macrocells due to damage from transverse impacts, cracks, and separations which, by locally depassivating the iron by lowering the pH and allowing depassivating substances to enter, will localize isolated anodic sites within the cathodic areas, which are the passivated surfaces where the coating still remains in intimate contact with the metallic structure. From that point on, as the ratio of the cathodic surface area to the anodic surface area is generally large, the current flowing in these macrocells may be high whenever any active depolarization of the cathodic area is possible.